The present invention relates generally to pickup cartridges of moving-coil type wherein a coil plate comprising an electrically insulating thin plate and coils of electroconductive film formed in spiral-shaped patterns on the thin plate is provided in a vibration system. More specifically, the invention relates to a moving-coil type pickup cartridge provided with a coil plate of a shape such that there is no influence due to leakage flux from the magnetic field producing section.
In general, among the moving-coil type stereo pickup cartridges known heretofore, there has been one having a vibration system of a construction wherein a square or cross-shaped core, around which a coil wire is wound, is fixed to the rear end of a cantilever. In another known pickup cartridge of this type, two armature links are provided on a cantilever and coils are provided by winding coil wire respectively around the ends of these links.
In each of these known pickup cartridges, however, the moving-coil assembly fixed to the cantilever, which comprises the coil winding and the core or the coil windings and the armature links, has a large mass. Therefore, the equivalent mass of the vibration system is large, whereby the characteristics particularly in the high-frequency range are poor, and signal pickup with good characteristics over a wide band cannot be achieved. If, in order to reduce the mass, the number of winding turns of the coils is decreased, the output will drop. Consequently, it has not been possible by means of moving-coil type pickup cartridge known heretofore to accomplish good signal pickup reproduction with high output, good signal to noise ratio, and, moreover, flat characteristics up to even a high-frequency range over a wide band.
Another difficulty encountered in the prior art has been that, since a magnetic material such as iron or permalloy has been used for the core or coil winding frame, the magnetostriction due to hysteresis and magnetic saturation is large. Still another difficulty has been that, since the coil assembly comprises coil wire wound around a winding frame, the thickness and volume of the coil structure are large. For this reason, the gap between the yoke and the pole piece in which the coil structure is interposed must be made large, whereby the magnetic conversion efficiency is poor. A further problem has been that the work of winding the coil wire around the winding frame has been laborious. Particularly in order to obtain a high value of the above mentioned magnetic conversion efficiency, it is necessary to reduce the thickness and volume of the coil structure thereby to decrease the above mentioned gap. For this purpose, a very fine wire (e.g., of a diameter of 10 microns) must be used for the coil wire, and this gives rise to difficulties in the coil winding work, risk of wire breakage, and lowering of work efficiency.
Furthermore, according to the concept of another known stereo pickup cartridge of moving-coil type, two coils formed by winding coil wire in the same plane in D-shape as a whole are mounted on a cantilever in a state wherein they are partly overlapping each other and in a position where they intersect the polar axis perpendicularly. In this pickup cartridge, however, since the mass of the coil is large, the equivalent mass of the vibration system is large, and particularly the characteristics at the higher frequencies are very poor, whereby the cartridge cannot be considered to be practicel. There is also a suggestion that these coils may be formed by printed circuits, but, with the above described coil arrangement, reduction to practice is difficult in any case. A pickup cartridge which embodies the above concept has not yet been reduced to practice and placed on the market.
Accordingly, the inventors have previously proposed a novel pickup cartridge of moving-coil type in which the above described difficulties have been overcome, and which has been reduced to practice. In this previously proposed pickup cartridge, a pair of coils are formed in the form of thin film and in a substantially hexagonal, vortex-shaped pattern on a thin glass substrate measuring, for example, 1 mm and 2 mm in length and breadth with a thickness of 50 .mu.m. This coil plate, which is of very light weight, for example, of the order of 0.25 mg., is mounted on the cantilever of the pickup cartridge. In the forming of the above described coil patterns, a thin film of a metal material of high electroconductivity such as, for example, nickel is first formed on both surfaces of the thin insulative substrate by a process such as evaporation deposition in a vacuum. Then parts of the metal film thus deposited are removed by a process such as photo-etching to leave the metal film in the spirally wound pattern of the coils. To the casing of the pickup cartridge are fixed a pair of opposed yoke pieces having a gap therebetween in which the above mentioned coils are inserted and a permanent magnet for producing a magnetic field in the gap by way of these yokes. A feature of this pickup cartridge is that the mass of the coil plate is very small, whereby signal picking up can be carried out with good characteristics up to and through the higher frequencies.
In the above described previously proposed moving-coil type stereo pickup cartridge, a pair of yoke pieces form a magnetic field in a gap therebetween in which a coil plate is interposed. Each of the yoke pieces has two edge parts which are interconnected mutually at right angle and are at angles of 45.degree.-45.degree. with respect to the direction perpendicular to the record disc. However, since the yoke pieces have a shape such that they confront only approximately one half of the respective coils of the above mentioned coil plate, only one half of each coil contributes to the current inducing action and the other half does not as the coil plate vibrates together with the cantilever. Furthermore, leakage flux occurs from the edges of the yolk pieces. For this reason, in the above mentioned other half of each coil not confronting the corresponding yolk piece, there is induced a current of a direction such as to cancel the current induced in the above mentioned original magnetic flux by the leakage flux. For these reasons, there have been problems in the prior art such as low output level, occurrence of some distortion, and poor linearity.